Space exploration began in the 1960s, and since then about five hundred people    into space.
A．have traveledB．traveledC．had traveled D．a(chǎn)re traveling
A
本題考查時(shí)態(tài)。根據(jù)時(shí)間狀語(yǔ)since then可知與完成時(shí)連用。句意：太空探索開(kāi)始于20世紀(jì)60年代，自從那以后大約有500人游覽過(guò)太空。
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